In the field of chemical analysis, the gas chromatograph (GC) has been a mainstay of chemists since the early 1950's. The basic gas chromatograph as shown in FIG. 1 includes four functional units: an inlet for sample material, a carrier gas supply, a separating column, and a detector for sensing the composition of the column effluent. A measured sample is introduced through the sample inlet and vaporized within the carrier gas prior to entry into the separating column. The sample inlet may include a septum or other structural components. The material to be sampled may be introduced via a syringe or other means. A variation of the gas chromatograph involves using a precision valve, shown in FIG. 2, to obtain the measured amount of sample material, initially in a liquid form, for testing within the chromatograph. The supply of carrier gas is routed into the valve to sweep the measured sample from the valve to an interior side of a septum (if used) upon the chromatograph, and then into the column. No matter what type of measuring technique or structure of the sample inlet is used, a liquid sample must be adequately vaporized before entering the separating column unit of the chromatograph.
In the chromatograph variation using a precision valve, the sample may be vaporized within the valve prior to entry of the sample inlet. The accuracy of sample size and the control of the sample temperature (control of vaporization) are both critical to accurate analysis. Detector readings are evaluated in combination with sample size to determine the concentration of materials within sample material. Non uniform sample size directly affects final concentration calculations. Wear within the valve body can affect the precision machined slots and passages in the valve which carry the measured sample, thus, affecting the uniformity of sample size.
Temperature control is critical to the successful analysis of the sample material in any GC apparatus. Temperature of the sample material at the inlet of the precision valve must be kept low enough to prevent pre-volatilization of the sample material. The volume of a partially volatilized sample varies from a non-volatilized sample, thus, pre-volatilization creates non-uniform sample sizes. Temperature at the outlet of the valve and between the valve and sample inlet must be maintained within a desired range in order to prevent inadvertent condensation of the sample after it has been volatilized. Condensation can prevent the separating column from functioning correctly for the sample material being analyzed and condensed material may remain in the column, sample inlet and/or transfer lines rendering future measurements inaccurate as well.
Numerous devices are marketed to provide valve arrangements for chromatographic devices. Sampling valves are generally either rotary valves or push-pull valves. Those marketed by Valco Instruments Co., Inc. of Houston, Tex., are representative of the types of valves currently available in the market. Insulated heater valve enclosures are combined with the valve and sometimes the sample inlet to control the temperature of the valve and sample independently of the column temperature. These heated valve enclosures are ovens, which are placed over the body of the valve or sample inlet and connected to a source of current to heat the elements within the oven. Sensors must be employed on these independent heater elements to monitor and adjust the temperature of the valve body to avoid overheating the valve body with the sample enclosed. FIG. 2 shows a known cylindrical heater element inserted within a sleeve which is wrapped around the valve body.
In each of the existing valves, despite the separate heater elements, there are opportunities for the sample to cool in transit from the valve body to the column of the GC especially when the heater elements are adjusted to avoid pre-volatilization conditions. A typical point of condensation is within tubing which runs from the valve to the sample inlet. What is desired is a valve which avoids undue wear and which can repeatedly and accurately capture a measured sample from an inlet stream of sample material, manipulate and maintain the temperature of the sample as desired, and deliver a vaporized sample to the sample inlet and separating column of a chromatograph.